My gas tank filler flange wasn't terrible but did need some straightening. I came up with this idea to tweak it, very similar to the expensive IH tool. But can be made by anyone with a welder and some scrap steel. The only trick is getting the right radius on the 2 welded on tabs, I used a pc. of 1/8" wall pipe that was very close to the size and hammered them to shape before welding. You do have to move it around the rim a few times to get it all. This worked out pretty good and didn't cost much.

Just a 2 pc. assembly with a 6"long pc. of 3/8 threaded rod and 2 nuts trapping the plate

Tip jig into tank

Move up into position and pull-hold upward

Put on top plate, washer, and spin a 3rd nut down snug. Tighten with wrench until it stops- loosen, reposition, and repeat.